This invention relates to frequency control circuits and more particularly to such circuits which are used to control the output frequency of an electronic inverter.
Inverter circuits such as those found in DC-link variable speed constant frequency (VSCF) power systems include a plurality of power pole switching elements which operate with a fixed switching pattern generated by a microprocessor or other digital circuit. Both systems utilize a crystal oscillator as the frequency reference to control the output frequency of the inverter to within, for example, less than 0.05% of 400 Hz. In certain applications such as aircraft power systems, it is desirable to operate the inverter in parallel with another source such as a ground power cart. To permit the momentary paralleling of a VSCF system with an external power source for non-interrupted power transfer, it is necessary to force the output of the VSCF inverter into synchronism with the external power source. A typical ground power source may be subject to large frequency swings such as .+-.5% of 400 Hz. To track the power swings, the VSCF inverter requires a variable frequency reference signal. Although voltage controlled crystal oscillators are available, they generally operate over a small range of, for example, .+-.0.1%. Non-crystal voltage controlled oscillators can be designed to have any frequency range. However, in the 3 to 6 MHz range, voltage controlled oscillators use varactor tuning diodes in resonant circuits to vary the frequency. These circuits use very small capacitors and inductors and, therefore, are sensitive to layout and construction restraints. They are also sensitive to varnish and moisture and must be temperature compensated. Therefore, voltage controlled oscillators for use in aircraft power supply applications are relatively complex and occupy a relatively large volume.